The present invention relates to a printer having a paper feed system for feeding printing paper to a print station, and a paper treatment system for subjecting the paper to an anti-cockle treatment.
In printers in which paper or similar media are used as recording media, a tendency of the paper to cockle may sometimes constitute a serious problem. The cockling phenomenon is related to the fact that paper and similar materials tend to absorb humidity from ambient air and to expand and contract in accordance with their humidity content. Typically, the expansion and contraction is unisotropic and is particularly pronounced in a direction in which the fibers of the paper are predominantly oriented. When there exists a gradient in humidity within the paper, then the more humid portion of the paper will expand more than the drier portion, which inevitably leads to the production of cockles or wrinkles.
In a typical setup of an ink jet printer, for example, especially a large format printer, the paper is intermittently advanced over a flat sheet support plate, while a carriage moves back and forth across the paper, and ink jet printheads mounted on the carriage are energized to eject droplets of ink onto the paper so as to form a printed image. Since the carriage moves with relatively high velocity, the ink droplets ejected onto the paper undergo a certain aberration and are deposited on the paper in a somewhat dislocated position. The amount of dislocation is proportional to the flight distance of the ink droplets. Thus, when cockles are present in the paper, the flight distance is non-uniform and, accordingly, the dislocation of the spots of ink on the paper also becomes non-uniform, so that the quality of the printed image is deteriorated.
In a hot melt ink jet printer, the ink is solid at room temperature and must be heated above its melting point, typically in the order of magnitude of 100° C., before of droplets of liquid ink can be jetted onto the paper. As a result, when the image is being printed, the paper will be heated by the high temperature of the ink, and part of the water that has been absorbed in the paper will evaporate. This creates a humidity gradient in the paper in the area of the print station, and the production of cockles is likely to occur.
Several measures can be conceived of in order to cope with this cockling phenomenon in a printer. For example, the paper may be heated in order to reduce its intrinsic humidity content; mechanical means may be employed for forcibly holding the paper flat on the sheet support plate, and/or shielding measures may be taken for preventing the paper from coming into contact with humid ambient air, especially in a stand-by condition of the printer. However, whichever measures are taken for mitigating the cockling phenomenon, such measures generally have a negative effect on the productivity of the printer and/or lead to an increased energy consumption.
US 2003/0137573 discloses an ink jet printer of the type described above, having a vacuum belt forming the paper feed system, and a paper treatment system comprising a heater arranged at the vacuum belt.
US 2004/041893 and EP 0 307 251 disclose printers having a humidity sensor and a heater controlled thereby in order to adjust the temperature of the paper in the print station.